The present invention relates to discharge lamps that are designed for dielectrically impeded discharges and are also designated as silent discharge lamps. Such discharge lamps have an electrode set for producing discharges in a discharge medium that is located in a discharge space of the lamp. Provided in this case between at least one part of the electrode set and the discharge medium is a dielectric layer that forms the dielectric impediment. In the case of lamps in which it is fixed whether the electrodes operate as cathodes or anodes, at least the anodes are dielectrically separated from the discharge medium.
Such lamps are prior art and have recently been enjoying increasing attention, chiefly because it is possible with the aid of a pulsed mode of operation (U.S. Pat. No. 5,604,410) to achieve relatively high efficiencies that make use as a source of visible light or as a UV radiator seem attractive for various fields of application. Of particular interest in this case are lamps in which the discharge space is located between two generally plane-parallel plates that are denoted below as base plate and as top plate. In this arrangement, at least the top plate is at least partially transparent, being capable, of course, of bearing on its side facing the discharge space a fluorescent material that is not itself transparent in the true sense. Such lamps with a plate-like design are of interest chiefly as flat discharge lamps, for example, for backlighting purposes in the case of displays, monitors and the like.
In order to ensure sufficient stability in the case of relatively large lamp formats, it is possible to use between the base plate and the top plate support elements that are located inside the discharge space and connect the base plate and the top plate to one another. In the outer region, the plates can be connected via a frame that encloses the discharge space and is not denoted here as a support element. The support elements shorten the bending length between the outer edges of the plates in the region of which the frame described can be provided, and thereby improve the stability of the lamp against bending loads and compressive loads. It is also to be borne in mind in this case that silent discharge lamps are frequently filled with a discharge medium exhibiting low pressure such that a generally relatively large part of the external atmospheric pressure bears on the plates.
Starting from this prior art, the invention is based on the technical problem of specifying a silent discharge lamp of the type described having improved support elements.
The invention therefore provides: a discharge lamp having a base plate, a top plate for the light exit, which is at least partially transparent, a discharge space between the base plate and the top plate for holding a discharge medium, an electrode set for producing dielectrically impeded discharges in the discharge medium, a dielectric layer between at least one part of the electrode set and the discharge medium, and at least one support element which produces a connection between the base plate and the top plate, characterized in that the support element is a supporting projection which is constructed as a unipartite component of the top plate, and the outer contour of the supporting projection tapers in the direction from the top plate toward the base plate in at least one cutting plane perpendicular to the base plate.
The invention further relates to a display device with such a discharge lamp and thus, for example, a flat display screen, a display or the like using LCD technology.
In a departure from the relevant prior art, in which the support elements are placed as separate glass balls between the plates, the invention firstly adopts the approach of constructing the support elements as integrated components of the top plate. These are thus projections of the top plate that are directed toward the base plate and are a unipartite component of the top plate. The top plate is preferably already produced with these projections with the aid of a suitable shaping method, for example thermoformed or compression-molded. However, the projections can also be integrally formed subsequently. It is, however, essential that when assembling the lamp the top plate has supporting projections that are designed in a unipartite fashion with it. The outlay on the positioning and fixing of separate support elements between the plates should then be eliminated during assembly of the lamp. However, for example, it can also be sensible for the purpose of fastening the support projections on the base plate to provide a connecting elementxe2x80x94made from solder glass, for examplexe2x80x94between the base plate and the supporting projections.
Furthermore, the invention is based on the idea that a unipartite construction of spacer elements with the base plate, which arises as development from the conventional supporting balls to be connected to the base plate is more unfavorable because the contact between the support elements and the plates produces shadows in the luminance distribution that impair the homogeneity. It has emerged that these shadows are more pronounced the smaller the distance of the contacts causing the shadows from the light-emitting plane of the top plate. It is therefore regarded as more favorable not actually to avoid such contacts completely, but to arrange them situated as deeply as possible, that is to say remote from the light-emitting side. By this means, the shadows merge to a greater extent in the luminance distribution of the lamp, particularly when diffusers or other elements homogenizing the luminance are also used on the top side of, or above, the top plate. The larger the distance between such diffusers and similar elements and the structures causing shadows, the more effectively it is possible to distribute the shadows extensively or resolve them again.
The invention also provides that the supporting projections extend out from the top plate toward the base plate in a tapering shape. The tapering is intended in this case to occur in at least one cross-sectional plane that runs through the supporting projection perpendicular to the base plate. In this arrangement the plates need not necessarily be plane and parallel to one another. The term xe2x80x9cperpendicularxe2x80x9d is to be understood locally, as appropriate. Tapering means that the extent of the supporting projection in the direction along the plates (xe2x80x9calongxe2x80x9d is also to be understood locally, as appropriate) in the cross-sectional surface in the region just above the base plate is smaller than in a region further removed from the base plate. The supporting projection is therefore intended to be narrower in the direction of the base plate. In this case, the tapering can preferably encompass the entire height of the supporting projection.
Various advantages, which need not necessarily be present in parallel, can be achieved through this shaping. Firstly, narrower supporting projections produce smaller shadow effects in the region of contact with the base plate. Secondly, in any case when individual localized discharge structures are produced over the base plate with the aid of the pulsed operating method, these discharge structures can be distributed over the base plate largely without being influenced by the supporting projections when the lamp is being designed. It is possible for this reason to achieve a good degree of homogeneity of the luminance and a relatively dense arrangement with the discharge structures. Thirdly, such forms of supporting points can also be important for optical properties of the top plate, as will become even clearer in the following course of the description.
The basic idea of the invention is directed to the structure of an individual supporting projection. Of course, the invention preferably relates to discharge lamps having a multiplicity of supporting projections. In particular, there is no mandatory aim of the invention to keep the number of supporting projections as low as possible, as is the case in the prior art. Rather, the invention is directed to those lamps in the case of which a relatively large number, for example a number comparable to the number of the individual discharge structures, of supporting projections are used. In this case, the above statements preferably hold for most or all of these supporting projections. However, in the most general sense only one supporting projection need be present or be provided according to the invention.
In order to avoid additional shadings and to utilize possible positive optical effects of the supporting projections, the latter preferably consist of an optically transparent material. However, they can in this case be coated entirely or partially with a fluorescent material, as is also the case with the remaining top plate. The supporting projections and the remainder of the top plate preferably consist of glass.
The shaping of the supporting projections is preferably designed such that not only is a cross-sectional plane with a tapering cross section produced, but, moreover, there is also no cross-sectional plane in which the supporting projection widens substantially in the direction of the base plate. When expressed in other words, this means that the outer surface of the supporting projections faces the discharge space of the base plate, in any case the important part of the outer surface. It is also possible for there to be individual regions of the outer surface that run perpendicular to the base plate, but not over a substantial part of the circumference of the supporting projections. In this case, the outer surface extends from the base plate up to the top plate, which means that it is not a small subregion of the outer surface that is involved here.
The outer surface of the supporting projection is intended to form in relation to a plane that cuts the supporting projection and runs at least locally parallel to the base plate between the top plate and the base plate, an angle of preferably at least 120xc2x0, better at least 130xc2x0 and, in the most favorable case, 140xc2x0 or more, this angle being defined in a cutting plane perpendicular to said plane and in the direction of the base plate. The angle thus refers, as an obtuse angle, to an outer surface of the supporting projection tipped toward the base plate. With such obliquely situated outer surfaces, space for the discharges can still be created in the vicinity of the underside of the supporting projection adjacent to the base plate, on the one hand, but on the other hand these oblique outer surfaces are important for possible optical functions of the supporting projections.
Specifically, when the supporting projections according to the invention are limited by the obliquely running outer surfaces described, by refraction of light impinging from the discharge space, or by appropriate alignment of the emission characteristics of a fluorescent layer on the outer surface, they ensure an alignment of light into the core region of the supporting projections. It is thereby possible to counteract the shadow produced by the contact with the base plate.
Furthermore, together with a pattern, prescribed by the electrode structure, of individual discharges it is possible to undertake an optimization to a luminance that is as homogeneous as possible in an overall design of the arrangement of supporting projections and of the discharge structure. In addition to the shadow effect of the contact between supporting projection and base plate, it has also specifically to be taken into account that the individual discharge structures typically burn not below, but between supporting projections. Consequently, the maxima of the UV generation are likewise situated between the supporting projections. As a result of the effect of optical deflection, the light can be brought partly from these regions into the regions of the supporting projections so as to produce a relatively homogeneous luminance on the top side of the top plate. Thus, the basic idea of the invention consists at this junction in a departure from the prior art in considering the supporting projections not as disturbances in the luminance, to be homogenized separately, of the discharge structure. Rather, in the case of the invention the supporting projections preferably assume an active role in the light distribution and are taken into account in the overall design exactly as is the discharge distribution, which is likewise inherently inhomogeneous. The aspect of the invention addressed here is brought out more vividly by the exemplary embodiments.
To the extent that this application talks of individual discharges or discharge structures, these statements relate, strictly speaking, to regions prescribed by the design of the lamp, in particular of the electrodes and the supporting projections, in which such individual discharge structures can burn. Depending on the operating state of the lamp, however, variously extended discharge structures are also conceivable in this case within these regions. Thus, the regions need not necessarily be filled entirely with a discharge structure. Above all, the desire can be to influence the size of the discharge structures in conjunction with dimming functions of the lamp. The statements in this application therefore relate to the regions which can be filled to the greatest extent with discharge structures. To the extent that electrode structures are provided for determining preferred positions of discharges, there will generally be a 1:1 correspondence with the discharge regions.
As already touched upon, the supporting projections are to taper in the direction of the base plate. It is optimal in this case when the supporting projections are as narrow as possible in the region of the contact with the base plate, the term xe2x80x9cnarrowxe2x80x9d being measured in relation to the other dimensions of the supporting projection. xe2x80x9cNarrowxe2x80x9d is in this case a path forming a small fraction, for example less than ⅓, xc2xc or ⅕ of a typical transverse dimension (along the plates) of the supporting projection, for example half the height of the discharge space. This narrowness should be present in this case in at least one direction, but preferably in two directions in the xe2x80x9clocalxe2x80x9d plane of the base plate. In other words, it can be a linearly narrow or approximately punctiform contact surface.
Very generally, even in the case of somewhat larger bearing surfaces in relation to the base plate, the supporting projections can run substantially like ribs along the top plate, or be limited to small regions in relation to the dimensions of the plates. In the first-named case, it is the linear contact surfaces that are the general concern for narrow contact surfaces, while in the second case it is the approximately punctiform ones. The rib-like supporting projections can have specific stabilization functions, for example they can provide the top plate with an improved bending strength in one direction. Furthermore, as will be explained in still further detail in the exemplary embodiments, they can also serve to separate specific regions in the discharge space slightly from one another, in order to influence the discharge distribution. Thus, together with the electrode structure they can define preferred locations for individual discharges and separate individual discharges from one another along identical electrodes. On the other hand, the supporting projections limited locally in two directions in the plane of the plate offer the possibility of minimized shadow effects, and are generally sufficient for the support function.
A preferred shape for locally limited supporting projections can therefore be formed by a cone or by a pyramid, in the case of which the vertex touches the base plate (and is possibly somewhat flattened off or rounded in the process). In principle, any desired basic shapes come into consideration for the cones and pyramids, that is to say surfaces limited with curves, polygonal surfaces or mixtures thereof. However, it is largely supporting projections without edges, that is to say cones, that are preferred, because the edges can lead to certain irregularities in the light distribution.
As already stated, an attempt is to be made to keep the contact surfaces between supporting projections and base plate as small as possible. Limits can exist in this case that are set by production methods (rounding in the case of glass shaping) or by the mechanical point loading of the base plate, so that rather than a supporting projection actually coming to bear xe2x80x9cin a pointed fashionxe2x80x9d against the base plate, there is a slight rounding or flattening off. As long as this rounding or flattening off is not of any substantial consequence in relation to the size dimensions of the supporting projection, the basic idea of the xe2x80x9cnarrownessxe2x80x9d is not thereby impaired.
However, a preferred feature of the invention is to keep the contact surface between the supporting projection and the base plate as small as possible by virtue of the fact that it results only from bearing by touching. In other words, instances of bonding, solder glass and the like, which would necessarily enlarge the contact surface somewhat, are to be dispensed with as far as possible. For the rest, such additions usually have the disadvantage that they release gases upon heating during lamp production so that extensive pumping operations are required to keep the discharge medium pure. Production is substantially simplified if, in accordance with the invention, such substances are dispensed with. However, it is not excluded in the case of bearing by touching that the supporting projections are pressed slightly into other layers that are required in any case, for example into reflection layers or fluorescent layers on the base plate. A similar statement can hold for a fluorescent coating of the supporting projections themselves.
This bearing purely by touching between supporting projections and base plate generally suffices for the targeted stabilization effect, because mechanical stresses pressing the plates away from one another do not occur, as a rule. This holds, in particular, for the case, which is of most interest technically in any case, in which the discharge lamp is operated with a discharge medium at low pressure. The supporting projections are then pressed against the base plate by the external overpressure. According to an additional aspect of the invention, a multiplicity of supporting projections are provided between the base plate and the top plate. The invention therefore differs additionally from the prior art, in which an attempt was made to use the smallest possible number of support elements. The inventors have verified that, given appropriately more frequent support, it is possible to use comparatively thin base plates and top plates such that it is possible to realize a substantial weight saving for the overall lamp. The overall weight of the lamp is, however, of substantial importance for many applications. Moreover, in the case of relatively light plates the mounting method and automatic mounting devices possibly required therefor can be rendered substantially more simple and less expensive. Moreover, it is of course possible to achieve improved stability with a larger number of supporting projections. Furthermore, the processing times during production are shortened, because thinner plate materials and therefore smaller thermal capacitances occur.
In this case, the supporting projections are to be arranged assigned to individual localized discharge regions in the discharge space. It is firstly to be established in this regard that the individual localized discharge structures have appeared with the already mentioned pulsed operating method even without this invention and were able to be permanently localized by creating preferred sites on the electrodes. However, the invention is not restricted to lamps with such preferred sites. Rather, it transpires that the invention itself results in preferred locations between the supporting projections for individual discharge regions, so that the conventional structures, for example nose-like projections on the cathodes, can also be less strongly pronounced. To the extent that individual discharge structures or regions can be produced between the supporting projections according to the invention independently of the possible pulsed operating method, the invention also relates thereto.
The assignment between supporting projections and individual discharge regions is to be present in the invention at least insofar as the individual discharge regions are respectively surrounded by identical patterns of directly adjacent supporting projections. This excludes, of course, discharge regions in the edge region of the discharge lamp, that is to say in the vicinity of the frame or the lateral closure of the discharge vessel. The aim in this case is to design the pattern of the directly adjacent supporting projections around a discharge region together with this discharge region so as to homogenize the luminance here as far as possible. The relatively large number of supporting projections then does not play a disadvantageous role for the homogeneity (compare the above explanations on the overall design of the discharge lamp). Of course, individual supporting projections can be directly adjacent to more than one discharge region, and this will even be the rule.
It is also preferred that the supporting projections for their part are surrounded as far as possible by the same pattern of directly adjacent discharge regions in each case.
Finally, it is preferably possible to find a plane through the discharge space between the base plate and top plate and a direction in this plane along which the supporting projections and the discharge regions alternate. The alternating row need not be a row alternating directly one after the other (according to the pattern ababab . . . ). Also included is a row in which two supporting projections or two discharge regions occur regularly one after another as long as each supporting projection and each discharge region has at least one discharge region or at least one supporting projection as its neighbor (that is to say, for example, abbabbabb . . . or aabbaabb . . . ).
They need not necessarily be strictly colinear in this direction of the alternating row, but can also be distributed in a somewhat zigzag fashion. It is preferred for a multiplicity of such rows which are parallel to one another to exist in this plane. It is also preferred for there to be in the plane a second direction which is not situated parallel to the first-named direction and along which there is likewise an alternating row of supporting projections and discharge regions. In this case, there is preferably both a set of parallel rows in the first direction and a further set of parallel rows in the second direction. Consequently, the overall result is a planar pattern of supporting projections and discharge regions of alternating design, for example a chessboard pattern.
Moreover, it is preferred in the above definition that the direction along which the alternating row results connects the centers of directly adjacent discharge regions or discharge regions which are at most situated next but one or the centers of directly adjacent support elements or support elements situated next but one. Furthermore, it is preferred in the case of strip-like electrodes for adjacent discharge regions situated on one strip side to be respectively separated by supporting projections.
As already explained, a main aspect of the invention consists in interpreting the supporting projections as a component of the discharge lamp that contributes to the homogenization of the luminance distribution. This aspect is particularly important given the preferably relatively large number of supporting projections. In this case, as long as they are surrounded by a sufficient number of individual discharges, shadowings, occurring regularly in principle, owing to supporting projections can be compensated just as well by diffusers or other homogenizing measures, as was the case in a conventional way for the few supporting projections used. Moreover, the supporting projections can, however, as already explained, also be used themselves to effect light control, for which purpose they consist of optically transmitting material. Although the supporting projections can also be provided with a fluorescent coating, they can also (by comparison with the remainder of the top plate) be wholly or partially free from fluorescent material, for example, they can subsequently be wiped free. They can additionally be brightened thereby, because the unavoidable extinction of the fluorescent layer is eliminated.
Finally, in the case of this invention preference is given to those discharge lamps that are designed for bipolar operation, in the case of which the electrodes therefore function alternately as anodes and as cathodes. Owing to a bipolar operation, the discharge structures, which are inherently generally asymmetric, are superimposed on one another to form a symmetrical distribution on average over time, for which reason the optical homogenization can be further improved.